Locating an anatomical cavity in a body, for example the body of a patient, is important, inter alia, for anaesthetics, in which it is often necessary for substances which have an anaesthetic action to be introduced into the anatomical cavity, such as for example the epidural cavity which is located in the vicinity of the spinal cord. To locate the anatomical cavity, it is generally known to use a hollow needle and a reservoir filled with an isotonic liquid or with a gas mixture, often an injection syringe with a displaceable plunger. In this case, the hollow needle is introduced into a patient's body, in the vicinity of the location where the cavity which is to be located is situated. The injection syringe is positioned at the free end of the needle, and the liquid or gas mixture can be injected from the injection syringe through the needle and can reach the body of the patient. The person who is handling the injection syringe with the needle, for example a physician, uses one hand to introduce the needle further into the body and uses the thumb of the other hand to exert pressure on the plunger of the injection syringe. The liquid will seek to flow out of the injection syringe via the open end of the needle, but in the process will be subject to a resistance from the tissue in which the point of the needle is situated. As a result, a certain force will have to be exerted on the plunger, and a pressure will be produced in the liquid. When the point of the needle reaches the anatomical cavity, the liquid flowing out of the needle is no longer subject to any resistance from surrounding tissue, and the pressure in the liquid drops. The person who is handling the assembly can feel this in the hand which he is using to operate the injection syringe. When the epidural cavity has been reached, a catheter is often introduced via the needle into the epidural cavity, so that, for example, an anaesthetic substance can be administered.
When the epidural cavity is being located, it is highly important that the point of the needle should not pass beyond this cavity, since there is then a risk of the spinal cavity or spinal cord behind it being affected, which may have particularly adverse results for the patient. Therefore, all kinds of methods and devices have been developed which make it easier to locate an anatomical cavity and reduce the risk of the point of the needle being introduced too far.
An example of a method and device of this type is known, inter alia from EP 0 538 259.
The known device comprises a hollow needle, a fluid-filled reservoir which is in communication with the needle, pump means for pressurizing the fluid, measuring means for creating a pressure-measurement signal which is related to the pressure prevailing in the fluid, signal-conversion means for converting the pressure-measurement signal created by the measuring means into a form which is suitable for further processing, and reproduction means for emitting an acoustic signal which is related to the pressure-measurement signal which has been converted by the signal-conversion means.
The needle of the known device is intended to be introduced into a body and is connected to the reservoir in the form of an injection syringe. The injection syringe contains a fluid in the form of an isotonic liquid. The needle and the injection syringe are in communication with one another via a T-shaped connector. The pressure-measuring means, which are used to detect and measure the pressure prevailing in the liquid in the injection syringe, are also connected to this T-shaped connector. The known device also comprises a processor for processing a pressure-measurement signal created by the pressure-measuring means, in order that the rate of pressure variation can always be determined, which pressure variation is primarily the consequence of the movement of the needle in the body. The pressure data provided by the processor and the pressure-measuring means are continuously compared with margins stored in the processor.
When the known device is being used, the starting point is a situation in which the point of the needle is already situated in the vicinity of the cavity which is to be located. The needle, the injection syringe and the pressure-measuring means are connected to one another by changing the position of a switch. In the injection syringe there is a plunger which functions as a pump means for displacing the liquid through the needle and thus produces pressure in the liquid. The pressure which is shown on the screen when there is no pressure being exerted on the plunger of the injection syringe is calibrated to zero. Then, the person who is handling the injection syringe and the needle brings the pressure in the liquid in the injection syringe to a defined level by exerting pressure on the plunger. During this process, he can read the level of the pressure from the screen at any time. When the liquid in the injection syringe has been brought to the required pressure, the person who is handling the device can move the needle towards the cavity in the body while using the pressure data displayed on the screen to carefully maintain a pressure on the plunger. In the process, the pressure in the liquid will vary. When the level of the pressure variation exceeds a minimum level stored in the processor and/or the pressure variation rate over a defined time period is within minimum margins stored in the processor, the warning means are activated and emit a first, acoustic warning signal via acoustic reproduction means. If the pressure can be restored by slightly displacing the plunger, without further displacement of the needle, the first acoustic warning signal will stop. On the other hand, if a more abrupt pressure variation occurs and the pressure cannot be restored by displacing the plunger, the warning means emit a second, acoustic warning signal, which clearly differs from the first warning signal. From the second warning signal, the person who is handling the device can infer that the point of the needle has reached the anatomical cavity and that he must stop moving the needle.
A drawback of the device and method which are known from EP 0 538 259 is that the acoustic warning signals are emitted on the basis of an interpretation of the pressure data by the processor. Although the person who is handling the device can see the instantaneous pressure on the screen and can also feel this information through the plunger, he will quickly become inclined to depend only upon his hearing and trust the acoustic warning signals. However, in practice this has not proven satisfactory, and consequently the person who is handling the device will also look at the screen, so that he loses sight of the needle.